1. Field of the Invention
The present invention relates to a method and an apparatus for detecting a polishing endpoint of a substrate having an insulating film, and more particularly to a method and an apparatus for detecting a polishing endpoint based on reflected light from a substrate.
2. Description of the Related Art
In fabrication processes of a semiconductor device, various kinds of materials are repeatedly deposited as films on a silicon wafer to form a multilayer structure. For the formation of such a multilayer structure, it is important to planarize a surface of a top layer. A polishing apparatus configured to perform chemical mechanical polishing (CMP) is used as one of techniques for achieving such planarization.
The polishing apparatus of this type includes, typically, a polishing table supporting a polishing pad thereon, a top ring for holding a substrate (a wafer with a film formed thereon), and a polishing liquid supply mechanism for supplying a polishing liquid onto the polishing pad. Polishing of a substrate is performed as follows. The top ring presses the substrate against the polishing pad, while the polishing liquid supply mechanism supplies the polishing liquid onto the polishing pad. In this state, the top ring and the polishing table are moved relative to each other to polish the substrate, thereby planarizing the film of the substrate. The polishing apparatus typically includes a polishing endpoint detection unit. This polishing endpoint detection unit is configured to determine a polishing endpoint from a time when the film is removed until a predetermined thickness is reached or when the film in its entirety is removed.
One example of such polishing endpoint detection unit is a so-called optical polishing endpoint detection apparatus, which is configured to apply a light to a surface of a substrate and determine a polishing endpoint based on information contained in the reflected light from the substrate. The optical polishing endpoint detection apparatus typically includes a light-applying section, a light-receiving section, and a spectroscope. The spectroscope decomposes the reflected light from the substrate according to wavelength and measures reflection intensity at each wavelength. This optical polishing endpoint detection apparatus is often used in polishing of a substrate having a light-transmittable film. For example, the Japanese laid-open patent publication No. 2004-154928 discloses a method in which intensity of reflected light from a substrate (i.e., reflection intensity) is subjected to certain processes for removing noise components to create a characteristic value and the polishing endpoint is detected from a distinctive point (a local maximum point or a local minimum point) of a temporal variation in the characteristic value.
The characteristic value created from the reflection intensity varies periodically with polishing time as shown in FIG. 1, and local maximum points and local minimum points appear alternately. This phenomenon is due to interference between light waves. Specifically, the light, applied to the substrate, is reflected off an interface between a medium and a film and an interface between the film and an underlying base layer of the film. The light waves reflected from these interfaces interfere with each other. The manner of interference between the light waves varies depending on the thickness of the film (i.e., a length of an optical path). Therefore, the intensity of the reflected light from the substrate (i.e., the reflection intensity) changes periodically in accordance with the thickness of the film. The reflection intensity can also be expressed as a reflectance.
As shown in FIG. 1, the above-described optical polishing endpoint detection apparatus counts the number of distinctive points (i.e., the local maximum points or local minimum points) of the variation in the characteristic value after the polishing process is started, and detects a point of time when the number of distinctive points has reached a preset value. Then, the polishing process is stopped after a predetermined period of time has elapsed from the detected point of time.
The characteristic value is an index (a spectral index) obtained based on the reflection intensity measured at each wavelength. Specifically, the characteristic value is given by the following equation (1):Characteristic value (Spectral Index)=ref(λ1)/(ref(λ1)+ref(λ2)+ . . . +ref(λk))  (1)
In this equation (1), λ represents a wavelength of the light, and ref (λk) represents a reflection intensity at a wavelength λk. The number of wavelengths λ to be used in calculation of the characteristic value is preferably two or three (i.e., k=2 or 3).
As can be seen from the equation (1), the reflection intensity is divided by the refection intensity. This operation can remove noise components contained in the reflection intensity. Therefore, the characteristic value with less noise components can be obtained. Instead of the characteristic value, the reflection intensity (or reflectance) itself may be monitored. In this case also, since the reflection intensity changes periodically according to the polishing time in the same manner as the graph shown in FIG. 1, the polishing endpoint can be detected based on the change in the reflection intensity.
In a polishing process for the purpose of exposing a lower film by polishing an upper film, it is customary to prepare a polishing liquid such that a polishing rate of the lower film is lower than that of the upper film. This is for preventing excessive-polishing of the lower film so as to stabilize the polishing process. However, when the polishing rate is low, the characteristic value (or the reflection intensity) does not fluctuate greatly, as shown in FIG. 2. As a result, the periodical change in the characteristic value is hardly observed and it is therefore difficult to detect the distinctive point (the local maximum point or local minimum point) of the characteristic value. Consequently, an accurate polishing endpoint detection cannot be achieved. In addition, since the fluctuation of the characteristic value (or the reflection intensity) is affected by the thickness of both the upper film and the lower film and the types of films, variation in the initial film thickness between substrates may cause an error of the polishing endpoint detection. Generally, the variation in the initial film thickness between substrates in each process lot is about ±10%. Such variation in the initial film thickness can cause an error of the polishing endpoint detection, because a relationship between the distinctive point of the characteristic value (or the reflection intensity) and the exposure point of the lower film may be altered due to the variation in the thickness of the lower film between substrates.